Electrical interconnect forming method

ABSTRACT

An electrical interconnect forming method. The electrical interconnect includes a first substrate comprising a first electrically conductive pad, a second substrate comprising a second electrically conductive pad, and an interconnect structure electrically and mechanically connecting the first electrically conductive pad to the second electrically conductive pad. The interconnect structure comprises a non-solder metallic core structure, a first solder structure, and a second solder structure. The first solder structure electrically and mechanically connects a first portion of the non-solder metallic core structure to the first electrically conductive pad. The second solder structure electrically and mechanically connects a second portion of the non-solder metallic core structure to the second electrically conductive pad.

This application is a divisional application claiming priority to Ser. No. 11/733,840, filed Apr. 11, 2007, now U.S. Pat. No. 7,786,001 issued Aug. 31, 2010.

FIELD OF THE INVENTION

The present invention relates to an electrical interconnect structure and associated method for forming an electrical interconnect structure.

BACKGROUND OF THE INVENTION

Connections between structures are typically unreliable and subject to failure. Accordingly, there exists a need in the art to overcome at least one of the deficiencies and limitations described herein above.

SUMMARY OF THE INVENTION

The present invention provides method for forming an electrical structure comprising:

providing a first substrate comprising a first electrically conductive pad, a second substrate comprising a second electrically conductive pad, a first transfer substrate comprising a first cavity, a first non-solder metallic core structure comprising a spherical shape, and a second non-solder metallic core structure comprising a spherical shape, wherein said first non-solder metallic core structure comprises a diameter that is less than a diameter of said first cavity, and wherein said second non-solder metallic core structure comprises a diameter that is less than a diameter of said first cavity;

forming a first solder structure on said first electrically conductive pad;

first dispensing said first non-solder metallic core structure into said first cavity within said first transfer substrate;

first positioning after said first dispensing, said first transfer substrate such that a first section of a surface of said first non-solder metallic core structure is in contact with said first solder structure;

first heating after said first positioning, said first non-solder metallic core structure to a temperature sufficient to cause said first solder structure to melt and form an electrical and mechanical connection between said first section of said surface of said first non-solder metallic core structure and said first electrically conductive pad;

removing after said first heating, said first transfer substrate from said non-solder metallic core structure;

applying a first underfill encapsulant layer to said first substrate;

forming a second solder structure on a second section of said surface of said first non-solder metallic core structure;

second dispensing said second non-solder metallic core structure into said first cavity within said first transfer substrate;

second positioning after said second dispensing, said first transfer substrate such that a first section of a surface of said second non-solder metallic core structure is in contact with said second solder structure;

second heating after said second positioning, said second non-solder metallic core structure to a temperature sufficient to cause said second solder structure to melt and form an electrical and mechanical connection between said second section of said surface of said first non-solder metallic core structure and said first section of said surface of said second non-solder metallic core structure;

removing after said second heating, said first transfer substrate from said second non-solder metallic core structure;

forming a third solder structure on said second electrically conductive pad;

third positioning said first substrate comprising said first non-solder metallic core and said second non-solder metallic core structure such that a second section of said surface of said second non-solder metallic core structure is in contact with said third solder structure; and

third heating after said third positioning, said second non-solder metallic core structure to a temperature sufficient to cause said second solder structure solder to melt and form an electrical and mechanical connection between said second section of said surface of said second non-solder metallic core structure and said second electrically conductive pad resulting in an electrical and mechanical connection between said first electrically conductive pad and said second electrically conductive pad.

The present invention provides a method for forming an electrical structure comprising:

providing a first substrate comprising a first electrically conductive pad and a second electrically conductive pad, a second substrate comprising third electrically conductive pad and a fourth electrically conductive pad, a first transfer substrate comprising a first cavity and a second cavity, and a non-solder metallic core structure comprising a spherical shape, wherein said non-solder metallic core structure comprises a diameter that is less than a diameter of said first cavity;

forming a first solder structure on said first electrically conductive pad;

forming a second solder structure on said second electrically conductive pad;

covering said second cavity with a film having an opening corresponding to said first cavity;

dispensing said non-solder metallic core structure into said first cavity within said first transfer substrate;

first positioning said first transfer substrate such that a first section of a surface of said non-solder metallic core structure is in contact with said first solder structure and said second cavity is aligned with said second solder structure;

first heating said non-solder metallic core structure to a temperature sufficient to cause said first solder structure to melt and form an electrical and mechanical connection between said first section of said surface of said non-solder metallic core structure and said first electrically conductive pad;

removing said first transfer substrate from said non-solder metallic core structure;

forming a third solder structure on said third electrically conductive pad;

forming a fourth solder structure on said fourth electrically conductive pad;

second positioning said first substrate comprising said non-solder metallic core structure such that a second section of said surface of said non-solder metallic core structure is in contact with said third solder structure and said second solder structure is in contact with said fourth solder structure;

second heating said non-solder metallic core structure to a temperature sufficient to cause said third solder structure solder to melt and form an electrical and mechanical connection between said second section of said surface of said non-solder metallic core structure and said third electrically conductive pad resulting in an electrical and mechanical connection between said first electrically conductive pad and said third electrically conductive pad; and

third heating said second solder structure and said fourth solder structure to a temperature sufficient to cause said second solder structure and said fourth solder structure to melt and form an electrical and mechanical connection between said second solder structure and said fourth solder structure resulting in an electrical and mechanical connection between said second electrically conductive pad and said fourth electrically conductive pad, wherein said second heating and said third heating are performed simultaneously.

The present invention advantageously provides a simple structure and associated method for forming connections between structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross sectional view of a first electrical structure, in accordance with embodiments of the present invention

FIG. 2 depicts a first alternative to FIG. 1 illustrating a cross sectional view of a second electrical structure, in accordance with embodiments of the present invention.

FIG. 3 depicts a first alternative to FIG. 2 illustrating a cross sectional view of a third electrical structure, in accordance with embodiments of the present invention.

FIG. 4 depicts a first alternative to FIG. 3 illustrating a cross sectional view of a fourth electrical structure, in accordance with embodiments of the present invention.

FIG. 5 illustrates a cross sectional view of a fifth electrical structure, in accordance with embodiments of the present invention.

FIG. 6 depicts a second alternative to FIG. 2 illustrating a cross sectional view of a sixth electrical structure, in accordance with embodiments of the present invention.

FIG. 7 depicts a second alternative to FIG. 1 illustrating a cross sectional view of an seventh electrical structure, in accordance with embodiments of the present invention.

FIG. 8 depicts a second alternative to FIG. 3 illustrating a cross sectional view of a eighth electrical structure, in accordance with embodiments of the present invention.

FIGS. 9A-9G illustrate a process for generating the electrical structure of FIG. 1, in accordance with embodiments of the present invention.

FIGS. 10A-10I illustrate a process for generating the electrical structures of FIG. 2, FIG. 3, and FIG. 5, in accordance with embodiments of the present invention.

FIGS. 11A-11F illustrate a process for generating the electrical structure of FIG. 4, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a cross sectional view of an electrical structure 2 a, in accordance with embodiments of the present invention. Electrical structure 2 a comprises a substrate 1, a substrate 4, and a plurality of interconnect structures 5 a. Substrate 1 comprises a plurality of electrically conductive pads 10. Each pad of electrically conductive pads 10 may be connected to wires or electrical components within substrate 1. Substrate 4 comprises a plurality of electrically conductive pads 12. Each pad of electrically conductive pads 12 may be connected to wires or electrical components within substrate 4. Substrate 1 may comprise, inter alia, a semiconductor device (e.g., an integrated circuit chip, a semiconductor wafer, etc), a chip carrier (organic or inorganic), a printed circuit board, etc. Substrate 4 may comprise, inter alia, a semiconductor device (e.g., an integrated circuit chip, a semiconductor wafer, etc), a chip carrier (organic or inorganic), a printed circuit board, etc. Each interconnect structure 5 a comprises a non-solder metallic (i.e., does not comprise any solder material) core structure 14 and a solder structure 6 a. Solder structure 6 a comprises solder. Solder is defined herein as a metal alloy comprising a low melting point (i.e., about 100 degrees Celsius to about 340 degrees Celsius) that is used to join metallic surfaces together without melting the metallic surfaces. Solder structure 6 a comprises a layer of solder that completely surrounds non-solder metallic core structure 14. Alternatively (i.e., instead of a layer of solder that completely surrounds non-solder metallic core structure 14), solder structure 6 a could comprise a first portion of solder 9 a attached to a top side 14 a of non-solder metallic core structure 14 and a second portion of solder 9 b attached to a bottom side 14 b of non-solder metallic core structure 14. Each non-solder metallic core structure 14 may comprise any conductive metallic material that does not comprise solder including, inter alia, copper, gold, nickel, etc. Each interconnect structure 5 a electrically and mechanically connects an electrically conductive pad 10 to an electrically conductive pad 12. Non-solder metallic core structure 14 comprises a cylindrical shape. Solder structure 6 a may comprise any solder material suitable for flip chip interconnections including, inter alia, an alloy of tin such as SnCu, SnAgCu, SnPb, etc.

FIG. 2 depicts a first alternative to FIG. 1 illustrating a cross-sectional view of an electrical structure 2 b, in accordance with embodiments of the present invention. Electrical structure 2 b comprises substrate 1, substrate 4, and a plurality of interconnect structures 5 b. In contrast with electrical structure 2 a of FIG. 1, electrical structure 2 b of FIG. 2 comprises a plurality of interconnect structures 5 b. Each of interconnect structures 5 b comprises a spherical non-solder (i.e., does not comprise any solder material) metallic core structure 17 and a solder structure 6 b. Each solder structure 6 b comprises a layer of solder that completely surrounds an associated non-solder metallic core structure 17. Additionally, each of interconnect structures 5 b may comprise an additional solder structure 6 d. Each solder structure 6 b electrically and mechanically connects the associated non-solder metallic core structure 17 to an associated electrically conductive pad 10. Each solder structure 6 d electrically and mechanically connects the associated non-solder metallic core structure 17 (i.e., thru solder structure 6 b) to an associated electrically conductive pad 12. The aforementioned connections result in each interconnect structure 5 b electrically and mechanically connecting an electrically conductive pad 10 to an associated electrically conductive pad 12. Optionally, two different types of solder materials may be used for solder structure 6 b and solder structure 6 d. For example, solder structure 6 b may comprise an AuSn solder material and solder structure 6 d may comprise a solder material such as, inter alia, SnAg, SnCu, SnAgCu, SnBi, etc. For first level area array interconnects, each non-solder metallic core structure 17 may comprise a diameter of about 25 microns to about 150 microns. For second level area array interconnects (e.g., a ball grid array (BGA)), each non-solder metallic core structure 17 may comprise a diameter of about 0.2 mm to about 1.5 mm. Each non-solder metallic core structure 17 may comprise a core of any conductive metallic material that does not comprise solder including, inter alia, copper, gold, nickel, etc. Additionally, each non-solder metallic core structure 17 may comprise an additional layer(s) of non-solder metallic materials (i.e., different from a material comprised by non-solder metallic core structure 17) surrounding (e.g., see layer 19 in FIG. 3, infra) non-solder metallic core structure 17. The additional layer(s) may comprise any conductive metallic material including, inter alia, nickel, gold, tin, etc.

FIG. 3 depicts a first alternative to FIG. 2 illustrating a cross sectional view of an electrical structure 2 c, in accordance with embodiments of the present invention. Electrical structure 2 c comprises substrate 1, substrate 4, and a plurality of interconnect structures 5 c. In contrast with electrical structure 2 b of FIG. 2, electrical structure 2 c of FIG. 3 comprises a plurality of interconnect structures 5 c. Each of interconnect structures 5 c comprises a non-solder metallic core structure 17, a solder structure 6 c, and a solder structure 6 d. Each solder structure 6 c electrically and mechanically connects an associated non-solder metallic core structure 17 to an associated electrically conductive pad 10. Each solder structure 6 d electrically and mechanically connects an associated non-solder metallic core structure 17 to an associated electrically conductive pad 12. The aforementioned connections result in each interconnect structure 5 c electrically and mechanically connecting an electrically conductive pad 10 to an associated electrically conductive pad 12. Optionally, two different types of solder materials may be used for solder structure 6 c and solder structure 6 d. For example, solder structure 6 c may comprise an AuSn solder material and solder structure 6 d may comprise a solder material such as, inter alia, SnAg, SnCu, SnAgCu, SnBi, etc. Each non-solder metallic core structure 17 may comprise a core of any conductive metallic material that does not comprise solder including, inter alia, copper, gold, nickel, etc. Additionally, each non-solder metallic core structure 17 may comprise an additional layer(s) 19 of non-solder metallic materials (i.e., different from a material comprised by non-solder metallic core structure 17) surrounding non-solder metallic core structure 17. Additional layer(s) 19 may comprise any conductive metallic material including, inter alia, nickel, gold, tin, etc.

FIG. 4 depicts a first alternative to FIG. 3 illustrating a cross sectional view of an electrical structure 2 d, in accordance with embodiments of the present invention. Electrical structure 2 d comprises substrate 1, substrate 4, and a plurality of interconnect structures 5 d. In contrast with electrical structure 2 c of FIG. 3, electrical structure 2 d of FIG. 4 comprises a plurality of interconnect structures 5 d. Each of interconnect structures 5 d comprises a non-solder metallic core structure 17 a, a non-solder metallic core structure 17 b, a solder structure 6 c, a solder structure 6 d, a solder structure 6 e. Additionally (i.e., optionally), electrical structure 2 d comprises an underfill encapsulant layer 25 a and an underfill encapsulant layer 25 b. Each solder structure 6 e electrically and mechanically connects a non-solder metallic core structure 17 a to an associated a non-solder metallic core structure 17 b. Each solder structure 6 c electrically and mechanically connects a non-solder metallic core structure 17 a to an associated electrically conductive pad 10. Each solder structure 6 d electrically and mechanically connects a non-solder metallic core structure 17 b to an associated electrically conductive pad 12. The aforementioned connections result in each interconnect structure 5 d electrically and mechanically connecting an electrically conductive pad 10 to an associated electrically conductive pad 12. Optionally, three different types of solder materials may be used for solder structure 6 c, solder structure 6 d, and solder structure 6 e. For example, solder structure 6 c may comprise an AuSn solder material, solder structure 6 d may comprise a solder material such as, inter alia, SnAg, SnCu, etc, and solder structure 6 e may comprise a solder material such as, inter alia, SnAgCu, SnBi, etc. Each non-solder metallic core structure 17 a and 17 b may comprise a core of any conductive metallic material that does not comprise solder including, inter alia, copper, gold, nickel, etc. Non-solder metallic core structure 17 a may comprise a first material (e.g., copper) and non-solder metallic core structure 17 b may comprise a second material (e.g., gold). Additionally, each non-solder metallic core structure 17 a and 17 b may comprise an additional layer(s) 19 of metallic materials (i.e., different from a material comprised by non-solder metallic core structure 17 a and 17 b) surrounding non-solder metallic core structure 17 a and 17 b. Additional layer(s) 19 may comprise any conductive metallic material including, inter alia, nickel, gold, tin, etc. Additionally, non-solder metallic core structure 17 a may comprise a layer(s) 19 comprising a different material from a layer(s) 19 on non-solder metallic core structure 17 b. Underfill encapsulant layer 25 a surrounds non-solder metallic core structures 17 a and is in contact with substrate 1. Underfill encapsulant layer 25 b surrounds non-solder metallic core structures 17 b and is in contact with substrate 4. Underfill encapsulant layer 25 a is in contact with underfill encapsulant layer 25 b. Underfill encapsulant layer 25 a may comprise a first material (e.g., a highly filled silica-epoxy composite adhesive) and underfill encapsulant layer 25 b may comprise a second and different material (e.g., a lightly filled silica-epoxy composite adhesive). Underfill encapsulant layer 25 a may comprise a first coefficient of thermal expansion (e.g., comprising a range of about 5-15 ppm/C) that is different (e.g., lower) from a second coefficient of thermal expansion (e.g., comprising a range of about 15.1-40 ppm/C) comprised by encapsulant layer 25 b. Underfill encapsulent layer 25 a may additionally comprise a filler 25 c dispersed throughout.

FIG. 5 illustrates a cross sectional view of an electrical structure 2 e, in accordance with embodiments of the present invention. Electrical structure 2 e in FIG. 5 is a combination of electrical structures 2 b and 2 c, of FIGS. 2-3. In addition to electrical structures 2 b and 2 c, of FIGS. 2-3, electrical structure 2 e in FIG. 5 comprises interconnection structures 29 (i.e., comprising solder) electrically and mechanically connecting some of electrically conductive pads 10 to associated electrically conductive pads 12. Therefore, electrical structure 2 e uses a combination of interconnect structures 5 b, 5 c, and 29 to electrically and mechanically connect electrically conductive pads 10 to associated electrically conductive pads 12. Note that any combination and any configuration of interconnect structures 5 b, 5 c, and 29 may be used to electrically and mechanically connect electrically conductive pads 10 to associated electrically conductive pads 12. For example, electrical structure 2 e may comprise only interconnect structures 5 c and 29 to electrically and mechanically connect electrically conductive pads 10 to associated electrically conductive pads 12. There may be any number or ratio of interconnect structures 5 b, 5 c, and 29 arranged in any pattern (e.g., interconnect structures 5 b and 29: may be placed such that they are in alternating positions, may be placed in random positions, may be placed such that there is one interconnect structure 5 b for every three interconnect structures 29, may be placed such that interconnect structures 5 b provide power and ground connections only and interconnect structures 29 are placed for signal interconnects only, etc). Additionally, electrical structure 2 e may comprise an underfill encapsulant layer 31 between substrate 1 and substrate 4.

FIG. 6 depicts a second alternative to FIG. 2 illustrating a cross sectional view of an electrical structure 2 f, in accordance with embodiments of the present invention. In contrast with electrical structure 2 b of FIG. 2, electrical structure 2 f of FIG. 6 comprises an underfill encapsulant layer 32 a between substrate 1 and substrate 4. In the case in which substrate 1 is a semiconductor device or a silicon wafer, underfill encapsulant layer 32 a may alternately comprise an underfill layer applied prior to chip joining or applied on the silicon wafer over the interconnect structures 5 b. Such an underfill layer is defined as a wafer-level underfill.

FIG. 7 depicts a second alternative to FIG. 1 illustrating a cross sectional view of an electrical structure 2 g, in accordance with embodiments of the present invention. In contrast with electrical structure 2 a of FIG. 1, electrical structure 2 g of FIG. 7 comprises an underfill encapsulant layer 32 b between substrate 1 and substrate 4. In the case in which substrate 1 is a semiconductor device or a silicon wafer, underfill encapsulant layer 32 b may alternately comprise an underfill layer applied prior to chip joining or applied on the silicon wafer over the interconnect structures 5 a. Such an underfill layer is defined as a wafer-level underfill

FIG. 8 depicts a second alternative to FIG. 3 illustrating a cross sectional view of an electrical structure 2 h, in accordance with embodiments of the present invention. In contrast with electrical structure 2 c of FIG. 3, electrical structure 2 h of FIG. 8 comprises an underfill encapsulant layer 32 c between substrate 1 and substrate 4. In the case in which substrate 1 is a semiconductor device or a silicon wafer, underfill encapsulant layer 32 c may alternately comprise an underfill layer applied prior to chip joining or applied on the silicon wafer over the interconnect structures 5 c. Such an underfill layer is defined as a wafer-level underfill.

FIGS. 9A-9G illustrate a process for generating electrical structure 2 a of FIG. 1, in accordance with embodiments of the present invention.

FIG. 9A illustrates a cross sectional view of a non-solder metallic layer 37 formed over an insulator layer 35, in accordance with embodiments of the present invention. Non-solder metallic layer 37 may comprise any non-solder metallic material such as, inter alia, copper, gold, nickel, etc. Insulator layer 35 may comprise any insulator material such as, inter alia, a polymer film (e.g., polyimide), etc.

FIG. 9B illustrates a cross sectional view of the structure of FIG. 9A after non-solder metallic interconnect structures 14 have been formed in order to form structure 35 a, in accordance with embodiments of the present invention. Non-solder metallic interconnect structures 14 may be formed by subtractively etching portions of non-solder metallic layer 37 (i.e., of FIG. 1) in order to form non-solder metallic interconnect structures 14. Non-solder metallic interconnect structures 14 may comprise various widths, heights, and height-to-width aspect ratios. A subtractive etching process comprises:

1. Applying and patterning a protective photo resist layer

2. Using chemical solutions to etch or dissolve unprotected regions of copper.

3. Stripping off the protective photo resist layer.

Each of non-solder metallic interconnect structures 14 may comprise a width of about 10 microns to about 100 microns and comprise a height-to-width aspect ratio of about 1:1 to about 5:1.

FIG. 9C illustrates a cross sectional view of substrate 1 of FIG. 1 after first portions of solder 9 a (i.e., solder structures) have been formed thereby forming a structure 35 b, in accordance with embodiments of the present invention. For example, substrate 1 may comprise a silicon device wafer that is prepared with electrically conductive interconnect pads (e.g., see pads 10 of FIG. 1). Solder is applied to the pads in order to form first portions of solder 9 a. Any method may be used to apply the solder to the electrically conductive interconnect pads, including, inter alia, applying solder as an injection molded solder.

FIG. 9D illustrates a cross sectional view of structure 35 a of FIG. 9B of FIG. 1 aligned with structure 35 b of FIG. 9C, in accordance with embodiments of the present invention. Non-solder metallic interconnect structures 14 are aligned to associated first portions of solder 9 a. The alignment process may comprise using commercially available bonding tools that use optical sensing of fiducials on substrate 1 and insulator layer 35.

FIG. 9E illustrates a cross sectional view of structure 35 c formed after the alignment process described with respect to FIG. 9D, in accordance with embodiments of the present invention. In FIG. 9E, a transfer process has been performed by heating the aligned assembly of FIG. 9D to a temperature above a melting point (i.e., with assistance of a fluxing agent or a fluxing atmosphere) of the solder used to form first portions of solder 9 a. Optionally, the transfer process may be assisted by a laser release process applied through a backside 21 of insulator layer 35. Light energy generated by a laser is absorbed by insulator layer 35 at an interface 23 to non-solder metallic interconnect structures 14 causes adhesion (i.e., at interface 23) to be degraded hereby releasing non-solder metallic interconnect structures 14 from insulator layer 35. Alternatively, an adhesive (i.e., at interface 23) may be degraded and release non-solder metallic interconnect structures 14 from insulator layer 35 during the solder melting process described, supra.

FIG. 9F illustrates a cross sectional view of a process for aligning structure 35 c of FIG. 9E with a structure 35 d, in accordance with embodiments of the present invention. Structure 35 d comprises a substrate 4 comprising formed solder structures 9 b (i.e., formed by a similar process to the process performed with respect to FIG. 9C).

FIG. 9G illustrates a completed electrical structure 35 e similar to electrical structure 2 a of FIG. 1, in accordance with embodiments of the present invention. An assembly of substrate 1 to substrate 4 through non-solder metallic interconnect structures 14, solder structures 9 a, and solder structures 9 b is carried out by raising a temperature of non-solder metallic interconnect structures 14 above a melting temperature of solder structures 9 b with the assistance of a fluxing agent or fluxing atmosphere. Optionally, non-solder metallic interconnect structures 14, solder structures 9 a, and solder structures 9 b may be encapsulated with polymeric material by capillary underfill following the joining of substrate 1 to substrate 4. Alternatively, an underfill encapsulant may be applied at wafer-level or on singulated devices prior to the joining of substrate 1 to substrate 4.

FIGS. 10A-10I illustrate a process for generating electrical structure 2 b of FIG. 2, electrical structure 2 c of FIG. 3, and electrical structure 2 e of FIG. 5, in accordance with embodiments of the present invention. Note that although FIGS. 10A-10I illustrate a process for applying solder as an injection molded solder, any solder applying process may be used.

FIG. 10A illustrates a cross sectional view of a structure 39 a comprising a filled glass or silicon mold 40 positioned over substrate 1 (i.e., from FIGS. 2 and 3), in accordance with embodiments of the present invention. Glass or silicon mold 40 is filled with solder that when released from glass mold will become solder structures 6 b of FIG. 2, 6 c of FIG. 3, and 6 b of FIG. 5. The solder may comprise any solder suitable for flip chip interconnects including, inter alia, an alloy of tin such as, inter alia, AuSn, SnCu, SnAgCu, etc. The solder may comprise a high melting point so that solder structures 6 b, 6 c will not melt during a subsequent step.

FIG. 10B illustrates a cross sectional view of a structure of 39 b formed from structure 39 a of FIG. 10A, in accordance with embodiments of the present invention. In FIG. 10B, the solder has been released from glass or silicon mold 40 to form solder structures 6 c attached to electrically conductive pads 10 on substrate 1.

FIG. 10C illustrates a cross sectional view of a transfer substrate 43 comprising a plurality of non-solder metallic core structures 17, in accordance with embodiments of the present invention. Non-solder metallic core structures 17 are positioned in cavities 43 a within transfer substrate 43. Each of cavities 43 a comprises similar dimensions as non-solder metallic core structures 17 with cavity positions corresponding to positions of associated solder structures 6 c on electrically conductive pads 10. Transfer substrate 43 may comprise, inter alia, glass, silicon, or any material used for injection molded solder molds, etc. Non-solder metallic core structures 17 may be dispensed into cavities 43 a as a slurry in a solvent such as, inter alia, water isopropanol, etc. The solvent may comprise an appropriate amount of flux to assist in the wetting of solder structures 6 c to non-solder metallic core structures 17. In a case in which non-solder metallic core structures 17 are coated with gold, flux is not necessary. Optionally, the solvent may additionally comprise a small amount of thermally degradable polymeric adhesive to aid in retaining non-solder metallic core structures 17 in cavities 43 a. Cavities 43 a are fabricated to a size that will only cause one non-solder metallic core structure 17 to fall into it during a dispensing of non-solder metallic core structures 17.

FIG. 10D illustrates a cross sectional view of transfer substrate 43 of FIG. 10C comprising a selected plurality of non-solder metallic core structures 17, in accordance with embodiments of the present invention. As an optional feature of the process, transfer substrate 43 may be covered with a polymeric film (i.e., not shown) with through-holes matching some pre-determined fraction of cavities 43 a. The pre-determined fraction of cavities 43 a covered by the polymeric film will be prevented from receiving non-solder metallic core structures 17. The pre-determined fraction of cavities 43 a allows a packaging design engineer to selectively place non-solder metallic core structures 17. Additionally, solder interconnects 29 may be selectively placed in some of cavities 43 a (i.e., instead of select non-solder metallic core structures 17) for placement on substrate 1. In this option, transfer substrate 43 may be couvered with a second polymeric film (i.e., not shown) with through-holes matching the remaining cavities 43 a. The cavities 43 a covered by the polymeric film will be prevented from receiving solder interconnects 29.

FIG. 10E illustrates a cross sectional view of substrate 1 of FIG. 10B positioned over transfer substrate 43 comprising non-solder metallic core structures 17, in accordance with embodiments of the present invention. Substrate 1 of FIG. 10B is positioned over transfer substrate 43 comprising non-solder metallic core structures 17 in order to transfer non-solder metallic core structures 17 to substrate 1.

FIG. 10F illustrates a cross sectional view of substrate 1 after non-solder metallic core structures 17 have been released from transfer substrate 43 and connected to solder structures 6 b, in accordance with embodiments of the present invention. In FIG. 10F, solder structures 6 b completely surround non-solder metallic core structures 17.

FIG. 10G depicts an alternative to FIG. 10F illustrating a cross sectional view of a structure 39 c comprising substrate 1 after non-solder metallic core structures 17 have been released from transfer substrate 43 and connected to solder structures 6 c, in accordance with embodiments of the present invention. In FIG. 10G, solder structures 6 c partially surround non-solder metallic core structures 17.

FIG. 10H illustrates a cross sectional view of substrate 1 positioned over substrate 4, in accordance with embodiments of the present invention. Substrate 1 is connected to substrate 4 in order to form electrical structure 2 b of FIG. 2.

FIG. 10I illustrates an alternative cross sectional view of substrate 1 positioned over substrate 4, in accordance with embodiments of the present invention. In the case in which the option of FIG. 10D is used (i.e., comprising solder interconnect structures 29), the positioning (not shown) is done similarly as in FIG. 10I. Substrate 1 is connected to substrate 4 in order to form electrical structure 2 c of FIG. 3.

FIGS. 11A-11F illustrate a process for generating electrical structure 2 d of FIG. 4, in accordance with embodiments of the present invention.

FIG. 11A illustrates structure 39 c of FIG. 10G comprising an underfill layer 25 a, in accordance with embodiments of the present invention. Structure 39 c in FIG. 11A has been formed by the process steps described with reference to FIGS. 10A-10E. Underfill layer 25 a may comprise a filler 25 c to create a low coefficient of thermal expansion (CTE). Underfill layer 25 a may comprise a coefficient of thermal expansion (CTE) similar to that of substrate 1.

FIG. 11B illustrates structure 39 c comprising a glass or silicon mold 40 b positioned over non-solder metallic core structures 17 a, in accordance with embodiments of the present invention. Glass or silicon mold 40 b is filled with solder that when released from mold 40 b will become solder structures 6 e of FIG. 4. The solder may comprise any solder suitable for flip chip interconnects including, inter alia, an alloy of tin such as, inter alia, AuSn, SnCu, SnAgCu, etc. The solder may comprise a high melting point so that solder structures 6 e will not melt during a subsequent step.

FIG. 11C illustrates a cross sectional view of structure of 39 c comprising solder structures 6 e attached to non-solder metallic core structures 17 a, in accordance with embodiments of the present invention. In FIG. 11C, the solder has been released from glass or silicon mold 40 b to form solder structures 6 e attached to non-solder metallic core structures 17 a.

FIG. 11D illustrates a cross sectional view of structure 39 c of FIG. 11C positioned over a transfer substrate 43 comprising non-solder metallic core structures 17 b, in accordance with embodiments of the present invention. Structure 39 c of FIG. 11C is positioned over transfer substrate 43 comprising non-solder metallic core structures 17 b in order to transfer and connect non-solder metallic core structures 17 b to non-solder metallic core structures 17 a.

FIG. 11E illustrates a cross sectional view of structure 39 c of FIG. 11D after non-solder metallic core structures 17 b have been connected to non-solder metallic core structures 17 a, in accordance with embodiments of the present invention.

FIG. 11F illustrates a cross sectional view of structure 39 c of FIG. 11E comprising an underfill layer 25 b applied over underfill layer 25 a, in accordance with embodiments of the present invention. After underfill layer 25 b is applied over underfill layer 25 a, substrate 1 is connected to substrate 4 in order to form electrical structure 2 d of FIG. 4.

While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention. 

What is claimed is:
 1. A method for forming an electrical structure comprising: providing a first substrate comprising a first electrically conductive pad, a second substrate comprising a second electrically conductive pad, and a first transfer substrate comprising a first cavity; providing a first non-solder metallic core structure comprising a spherical shape and a second non-solder metallic core structure comprising a spherical shape, wherein said first non-solder metallic core structure comprises a diameter that is less than a diameter of said first cavity, and wherein said second non-solder metallic core structure comprises a diameter that is less than a diameter of said first cavity; forming a first solder structure on said first electrically conductive pad; first dispensing said first non-solder metallic core structure into said first cavity within said first transfer substrate; first positioning after said first dispensing, said first transfer substrate such that a first section of a surface of said first non-solder metallic core structure is in contact with said first solder structure; first heating after said first positioning, said first non-solder metallic core structure to a temperature sufficient to cause said first solder structure to melt and form an electrical and mechanical connection between said first section of said surface of said first non-solder metallic core structure and said first electrically conductive pad; removing after said first heating, said first transfer substrate from said first non-solder metallic core structure; applying a first underfill encapsulant layer to said first substrate; forming a second solder structure on a second section of said surface of said first non-solder metallic core structure; second dispensing said second non-solder metallic core structure into said first cavity within said first transfer substrate; second positioning after said second dispensing, said first transfer substrate such that a first section of a surface of said second non-solder metallic core structure is in contact with said second solder structure; second heating after said second positioning, said second non-solder metallic core structure to a temperature sufficient to cause said second solder structure to melt and form an electrical and mechanical connection between said second section of said surface of said first non-solder metallic core structure and said first section of said surface of said second non-solder metallic core structure; removing after said second heating, said first transfer substrate from said second non-solder metallic core structure; forming a third solder structure on said second electrically conductive pad; third positioning said first substrate comprising said first non-solder metallic core and said second non-solder metallic core structure such that a second section of said surface of said second non-solder metallic core structure is in contact with said third solder structure; and third heating after said third positioning, said second non-solder metallic core structure to a temperature sufficient to cause said second solder structure solder to melt and form an electrical and mechanical connection between said second section of said surface of said second non-solder metallic core structure and said second electrically conductive pad resulting in an electrical and mechanical connection between said first electrically conductive pad and said second electrically conductive pad.
 2. The method of claim 1 wherein said forming said first solder structure comprises applying a first portion of molten solder to said first electrically conductive pad from a second transfer substrate comprising a second cavity filled with said first portion of molten solder, wherein said forming said second solder structure comprises applying a second portion of molten solder to said second section of said surface of said first non-solder metallic core structure from said second transfer substrate comprising a third cavity filled with said second portion of molten solder, and wherein said forming said third solder structure comprises applying a third portion of molten solder to said second electrically conductive pad from said second transfer substrate comprising a fourth cavity filled with said third portion of molten solder.
 3. The method of claim 1, wherein said dispensing said first non-solder metallic core structure into said first cavity within said first transfer substrate comprises dispersing said first non-solder metallic core structure into a first liquid medium, wherein said dispensing said second non-solder metallic core structure into said first cavity within said first transfer substrate comprises dispersing said second non-solder metallic core structure into a second liquid medium, and wherein said first liquid medium and said second liquid medium each comprise a liquid selected from the group consisting of a fluxing agent, water, isopropanol, and a thermally degradable soluble polymeric adhesive.
 4. The method of claim 1, wherein said first positioning, said second positioning, and said third positioning comprises using a gaseous fluxing agent to assist in a solder wetting process.
 5. The method of claim 1, further comprising: after said removing said first transfer substrate from said second non-solder metallic core structure, applying a second underfill encapsulant layer to said first underfill encapsulant layer.
 6. The method of claim 1, wherein said first underfill encapsulant layer comprises a material that differs from a material comprised by said second underfill encapsulant layer.
 7. The method of claim 1, wherein said first underfill encapsulant layer comprises a first coefficient of thermal expansion, wherein said second underfill encapsulant layer comprises a second coefficient of thermal expansion, and wherein said second coefficient of thermal expansion is greater than said first coefficient of thermal expansion.
 8. The method of claim 1, wherein said first substrate comprises a substrate selected from the group consisting of a semiconductor device, a chip carrier, and a printed circuit board.
 9. The method of claim 1, wherein said second substrate comprises a substrate selected from the group consisting of a semiconductor device, a chip carrier, and a printed circuit board.
 10. A method for forming an electrical structure comprising: providing a first substrate comprising a first electrically conductive pad and a second electrically conductive pad, a second substrate comprising third electrically conductive pad and a fourth electrically conductive pad, a first transfer substrate comprising a first cavity and a second cavity, and a non-solder metallic core structure comprising a spherical shape, wherein said non-solder metallic core structure comprises a diameter that is less than a diameter of said first cavity; forming a first solder structure on said first electrically conductive pad; forming a second solder structure on said second electrically conductive pad; covering said second cavity with a film having an opening corresponding to said first cavity; dispensing said non-solder metallic core structure into said first cavity within said first transfer substrate; first positioning said first transfer substrate such that a first section of a surface of said non-solder metallic core structure is in contact with said first solder structure and said second cavity is aligned with said second solder structure; first heating said non-solder metallic core structure to a temperature sufficient to cause said first solder structure to melt and form an electrical and mechanical connection between said first section of said surface of said non-solder metallic core structure and said first electrically conductive pad; removing said first transfer substrate from said non-solder metallic core structure; forming a third solder structure on said third electrically conductive pad; forming a fourth solder structure on said fourth electrically conductive pad; second positioning said first substrate comprising said non-solder metallic core structure such that a second section of said surface of said non-solder metallic core structure is in contact with said third solder structure and said second solder structure is in contact with said fourth solder structure; second heating said non-solder metallic core structure to a temperature sufficient to cause said third solder structure solder to melt and form an electrical and mechanical connection between said second section of said surface of said non-solder metallic core structure and said third electrically conductive pad resulting in an electrical and mechanical connection between said first electrically conductive pad and said third electrically conductive pad; and third heating said second solder structure and said fourth solder structure to a temperature sufficient to cause said second solder structure and said fourth solder structure to melt and form an electrical and mechanical connection between said second solder structure and said fourth solder structure resulting in an electrical and mechanical connection between said second electrically conductive pad and said fourth electrically conductive pad, wherein said second heating and said third heating are performed simultaneously.
 11. The method of claim 10, wherein said forming said first solder structure comprises applying a first portion of molten solder to said first electrically conductive pad from a second transfer substrate comprising a third cavity filled with said first portion of molten solder, wherein said forming said second solder structure comprises applying a second portion of molten solder to said second electrically conductive pad from said second transfer substrate comprising a fourth cavity filled with said second portion of molten solder, wherein said forming said third solder structure comprises applying a third portion of molten solder to said third electrically conductive pad from a third transfer substrate comprising a fifth cavity filled with said third portion of molten solder, and wherein said forming said fourth solder structure comprises applying a fourth portion of molten solder to said fourth electrically conductive pad from said third transfer substrate comprising a sixth cavity filled with said fourth portion of molten solder.
 12. The method of claim 10, wherein said dispensing said non-solder metallic core structure into said first cavity within said first transfer substrate comprises dispersing said non-solder metallic core structure into a liquid medium, and wherein said liquid medium comprises a liquid selected from the group consisting of a fluxing agent, water, isopropanol, and a thermally degradable soluble polymeric adhesive.
 13. The method of claim 10, wherein said first positioning and said second positioning comprises using a gaseous fluxing agent to assist in a solder wetting process.
 14. The method of claim 10, further comprising: applying a first underfill encapsulant layer to said first substrate.
 15. The method of claim 14, further comprising: applying a second underfill encapsulant layer to said first underfill encapsulant layer.
 16. The method of claim 15, wherein said first underfill encapsulant layer comprises a material that differs from a material comprised by said second underfill encapsulant layer.
 17. The method of claim 16, wherein said first underfill encapsulant layer comprises a first coefficient of thermal expansion, wherein said second underfill encapsulant layer comprises a second coefficient of thermal expansion, and wherein said second coefficient of thermal expansion is greater than said first coefficient of thermal expansion.
 18. The method of claim 10, wherein said first substrate comprises a substrate selected from the group consisting of a semiconductor device, a chip carrier, and a printed circuit board.
 19. The method of claim 10, wherein said second substrate comprises a substrate selected from the group consisting of a semiconductor device, a chip carrier, and a printed circuit board. 